Atomic beam device

ABSTRACT

A cesium beam tube for use as a frequency standard, wherein the beam path from the cesium beam source unit to the detector is hermetically sealed. 
     This cesium beam tube includes a rigid high frequency transition part composed of a metallic block which is used as the basic support of the tube. A first state selection magnet part and the cesium beam generator are coaxially coupled to the one end of the rigid cavity, while the second state selection magnet part and detector are coaxially connected to the other end of the rigid cavity. The rigid cavity, first and second magnet poles, cesium beam generator and the detector form the vacuum envelope by themselves, and the permanent magnets and other related parts are not in a vacuum.

BACKGROUND OF THE INVENTION

This invention relates to an atomic beam device for frequency standardsusing an atomic or molecular beam, and in more detail, to a newlydesigned compact, rigid and inexpensive cesium beam tube which is easyto maintain.

An atomic beam tube which utilizes the atom spectrum of cesium (Cs) fora frequency standard generally includes the basic structure as shown inFIG. 1.

In FIG. 1, the cesium oven 1 which is used as the cesium beam sourceheats the cesium Cs to a temperature of 80° to 100° C. so that it isvaporized and thereby generates the Cs beam through the collimator.

This Cs beam enters the detector 7 through the first state selectionmagnet 2 for producing a magnetic field A, high frequency transitionpart 3, including the microwave cavity 5 placed in the field, forproducing a magnetic field C, and the second state selection magnet 6for producing a magnetic field B. On the other hand, a microwave signalis supplied to an RF input circuit 8. When the frequency of thismicrowave signal coincides with the transition frequency of the Csatoms, resonance of the Cs atoms occurs in the cavity 5, resulting inthe maximum output of the detector 7. Therefore, a highly stabilizedoscillation frequency can be obtained by using closed loop control forthe oscillation frequency of the microwave oscillator, for example, suchas a crystal controlled oscillator, so that the microwave signalfrequency is maintained at the center of the resonance spectrum of theCs atom. The transition frequency of the Cs atom in the ground state is9192.631770 MHz.

An example of the conventional cesium beam tube which is configurated bycombining the above-mentioned basic elements is shown in the Japanesepatent publication No. Toku-Ko-Sho 42-27517 (corresponding U.S. Pat. No.3,323,008). Moreover, an example of another conventional cesium beamtube is also disclosed in the Japanese patent laid-open application No.Toku-Kai-Sho 51-64895 (corresponding U.S. Pat. No. 3,967,115). Thestructure of the cesium beam tube in accordance with these prior artreferences is shown in FIG. 2 in the form of a cross-section.

In FIG. 2, the atomic beam generator 1, A magnetic field unit 2, highfrequency transition part 3, B magnetic field unit 6, detector 7, Cmagnetic field unit 4 and magnetic shield 10 are rigidly and levellymounted on the mount 9 as shown in FIG. 2. Each element is arranged onthe mount 9 so that the atomic beam generated from the atomic beamgenerator 1 reaches the detector through the specified route G. Themount 9, which is holding each element mentioned above, is provided atthe inside of the case 11 which ensures a hermetically sealed structurein combination with the cover 13. The high frequency input circuit 8 isengaged with the case 11 and projected to the outside, and moreover isheld to the mount 9 at the section D thereby resulting in a sufficientseal. In addition, at the above-mentioned high frequency signal inputport 8, which is coupled to the microwave signal to be supplied from theexternal circuit, a sealing material 14 is provided, through which saidmicrowave signal effectively passes. In order to hermetically seal eachelement accommodated in the case 11, the cover 13 is mounted to the case11 and the area around the joint, namely the part E, is sealed by ameans such as welding. In the above-mentioned structure of the atomicbeam device, an air exhaust system and vacuum ion pump are connected toan air exhaust port which is provided on case 11 but which is notillustrated in FIG. 2, so that the inside of the atomic beam device ismaintained in a vacuum condition. In this case, the ion pump may bebuilt inside of the device or provided as an additional unit outside ofthe device. After the air in the atomic beam device is exhausted until avacuum is obtained, the air exhaust system is removed. Then, by sealingsaid air exhaust port, the atomic beam device can be completed. The part12 is an airtight electrical connection terminal engaged with the case11, and is connected to the related portions of the atomic beam device.

However, the existing atomic beam device having the structure mentionedabove has always been accompanied by the following problems because therequired elements are all housed in the vacuum envelope. The magnetswhich form the magnetic fields A and B are permanent magnets which aregenerally manufactured by a method such as casting or sintering.Therefore, such magnets have many fine vacant spaces, which are filledwith various kinds of gas. Since it is difficult to sufficiently exhaustgas when sealing the atomic beam device in the vacuum condition, the gasmay gradually be released to the inside of the atomic beam device afterit is completed. If such gas is released in the atomic beam device, thevacuum is deteriorated, and moreover the Cs source or other portions maybe contaminated. Furthermore, the excitation coil used for producingmagnetic field C and other internal wiring with an insulation coatingalso absorbed various kinds of gas, causing the disadvantages mentionedabove.

When exhausting the air from the atomic beam device, it is necessary toexhaust the air while the tube is heated to a specified bakingtemperature in order to exhaust the above-mentioned absorbed gas. Thisbaking temperature is usually 300° C. or higher, which deteriorates ordemagnetizes the permanent magnets provided for the magnetic fields Aand B. Moreover, the excitation coil for the magnetic field C and theinsulation for covering other wires must be heat-resistant. In addition,since the highly permeable magnetic material used as the yoke for saidmagnetic field C may, for example, be mechanically held together bymeans of small screws, stress may be generated inside of the yokematerial due to differences in the thermal expansion coefficients duringsaid baking, thereby sometimes generating distortion. Such distortion,if it occurs, will result in a non-uniform magnetic field C applied tothe atomic beam path of the high frequency transition part. For thisreason, it was difficult to exhaust the air from the atomic beam devicewhile maintaining an adequate baking temperature.

Moreover, the air should be exhausted by sustaining the atomic beamdevice at the baking temperature for a considerable period of time. Onthe other hand, as described previously, the gas released gradually fromthe magnets, coil, wiring and other materials even after the air isexhausted deteriorates the vacuum or contaminates the environment, so itwas also necessary to sustain the vacuum by additionally providing anion pump. Said ion pump is positioned outside the atomic beam device orbuilt inside of said device. The former method increases the outsidedimensions, while the latter method inevitably results in a complicateddevice because of the magnet of the ion pump. Furthermore, regardless ofhow the ion pump is installed, a high voltage power supply is requiredin order to operate said ion pump, complicating the structure of thedevice. An extra power supply and facilitates are required for operatingthe atomic beam device, raising the cost as a whole.

Since each component of the existing atomic beam device is supported onthe standard level F of the mount 9, which is spaced far from the atomicbeam path G, minute displacements such as twisting or inclinationbetween respective elements influence the atomic beam path G to anundesirably large extent. In order to prevent this disadvantage, it isnecessary for mount 9 to be strong enough for maintaining mechanicalflatness, so that the material used must have rigidity and sufficientstrength. However, particularly if the device is shipped on vehicles,ships or airplanes, a specially designed buffer unit must be added,thereby increasing weight and size, so that the device will surviveexternal forces such as vibration, impact and inertia etc.

SUMMARY OF THE INVENTION

It is a main object of the present invention to offer a new version ofthe atomic beam device which solves the disadvantages in the existingatomic beam device, particularly a cesium beam tube.

It is another object of the present invention to offer an improvedatomic beam device having compact design, light weight, and rigidstructure.

Moreover, another object of the present invention is to offer an atomicbeam device which has a coaxial supporting structure with excellentmechanical strength, and which can be assembled easily with little errorin the parts arrangement.

It is a further object of the present invention to offer an improvedatomic beam device which eliminates the necessity of a vacuum ion pumpand which allows little deterioration in the vacuum.

In summary, the present invention is characterized, in an atomic beamdevice having an atomic beam generator, a first state selection magnetpart, a high frequency transition part, a second state selection magnetpart and a detector, by hermetically coupling each element mentionedabove on the common axis of the beam path.

In addition, according to other characteristics of the presentinvention, said high frequency transition part is composed of a metalblock forming a microwave cavity, and the structure using this metalblock as the basic supporting body of the atomic beam device is given.At one end of the metal block, which forms a rigid cavity, the firststate selection magnet portion and the Cs chamber for generating theatomic beam are coaxially coupled in the sealed condition, while at theother end of the metallic block, the second state selection magnetportion and the detector are coaxially coupled also in the sealedcondition. The permanent magnets for the magnetic fields A and B and thecoil for the magnetic field C are provided outside of the vacuum beampath.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a principle structure of an atomic beam device;

FIG. 2 shows a cross section of the existing cesium beam devicestructure;

FIG. 3 shows a cross section of an embodiment of the cesium beam devicerelated to the present invention;

FIG. 4 shows a perspective view of the metal block of the high frequencytransition part providing a microwave cavity;

FIG. 5 shows a perspective view of the magnetic unit structures used forthe magnetic fields A and B;

FIG. 6 is an outline of the cesium beam oven; and

FIG. 7 shows a cross section along the line 7--7 of FIG. 3 of thestructure for establishing the magnetic field C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a cesium beam device according to the presentinvention will be explained hereunder by referring to FIG. 3.

In FIG. 3, the high frequency transition part 3 includes a metal block301 formed of two block portions 301' as shown in FIG. 4. Said metalblock is manufactured from, for example, oxygen-free copper. Namely, ahalf of the branching waveguide 302 and the inlet and outlet ports 303for the atomic beam are formed in each of the two portions 301', theadjoining surfaces of which are parallel to the atomic beam patch inFIG. 4. The chamber 304 is formed simultaneously by cutting or pressingbetween the inlet and outlet ports 303. Getter for absorbing Cs or othergases through a coating of graphite, for example, may be deposited onthe internal wall of chamber 304. Further, in case it is necessary tomeasure the magnetic field, it is also possible to include a Zeeman coilin chamber 304. One portion 301' of block 301 is combined with the otherportion 301' (illustrated by the dot-dash line in FIG. 4), which ismachined quite symmetrically and hermetically sealed by soldering at thejoint where mutual waveguides are positioned face to face. At the highfrequency input port 8, the coupling flange 801 and high frequencywaveguide 802 are hermetically sealed at the specified position bysoldering so that they are integrated into said block. At the highfrequency input port 8, the sealing window 803, which is made ofdielectric material, is welded in order to sustain the waveguide 302 inthe sealed condition. Said window is generally well known. The surfacesH--H of block 301, which are parallel to the atomic beam path, are usedas the standard level for fixing the yoke for establishing the magneticfield C at a constant space as will be described later. Both ends I--I,which are perpendicular to the atomic beam path, are respectively usedfor soldering the flanges 305 and moreover become the standard surfacesfor placing the magnetic units for fields A and B, the Cs atom beamgenerator 1, and detector 7 at the proper locations. The flanges 305,which may be mounted by soldering, have central hollowed areas which arerespectively engaged with the surfaces I of the block 301, while at theopposite sides are hollowed areas which engage the magnetic units,including the yokes for non-uniform magnetic fields for the magneticfields A and B. Each flange 305 has a central hole which is required inorder to form the path of the atomic beam.

The magnetic units which form the poles for the magnetic fields A and Bare symmetrically arranged at both sides of the high frequencytransition part 3 using structures such as shown in the perspective viewof FIG. 5. It is noted that the suffixes "A" and "B" in FIG. 3 identifyelements associated with fields A and B, respectively. At the atomicbeam path, the poles 201 and 202 are arranged face to face in order toform a non-uniform magnetic field and the spacers 203 are arranged atboth sides in order to maintain the space between said poles. Thespacers 203 are made of a non-magnetic material. The metallic cylinders204 and 205 extend from the ends of the magnetic units and are mountedin the hermetically sealed condition. Said poles 201 and 202, spacers203, and cylinders 204 and 205 are assembled at both ends of the metalblock 301 in the positions illustrated and then sealed at every joint bysoldering.

The metal cylinders 205 of both magnetic units are made of oxygen-freecopper allowing plastic deformation to facilitate adjustment of the beamaxis, as will be described later, and sealed by soldering after beingengaged with the outer hollowed areas of the flanges 305. Here, beforethe above-mentioned soldering, the flanges 206A and 206B are welded tothe collars J provided at the outside of cylinders 205A and 205B asshown in FIG. 3. The end of cylinder 204 of the magnetic field unit formagnetic field A is sealed to the atom beam generator 1. Moreover, theend of cylinder 204 of the magnetic field unit for field B is sealed tothe detector 7.

FIG. 6 shows a perspective view of the atomic beam generator 1. Theflange 101 is integrated to the cylinder 204 of said magnetic field Aunit. Chamber 102, which has a diaphragm and a needle for breaking asealed vessel containing cesium, is mechanically coupled to flange 101by means of a metal support 103 made of the material which allows littlepropagation of heat. Moreover, metal support 103 is configured tominimize heat propagation. That is, heat propagation is reduced bycutting away portions to leave a plurality of feet attached to flange101.

The structure and functions of the atom beam generator 1 will beexplained in more detail by referring again to FIG. 3. In FIG. 3, insideof the chamber 102, is a sealed vessel 104 containing cesium. A needle105 for unsealing the vessel is also provided inside of the diaphragmopposite said sealed vessel.

Flange 101 is connected to case 107 of the unsealing mechanism for thevessel 104 by means of the spacer ring 106. The unsealing means for saidvessel comprises the bellows 108, which can be extended inwardly fromthe end of case 107, and the end boss 109, which is internally screwedand mounted at the end of said bellows 108. Said spacer ring 106, case107, and bellows 108 and boss 109 are respectively sealed by solderingat the contact areas. Tightly engaged screw 110 extends from a hole atthe center of the end of case 107 to boss 109. When the screw 110 isrotated into case 107, the bellows 108 extend up to the diaphragmsurface of chamber 102. When the screw is rotated further, the boss 109comes to contact with the diaphragm surface of the chamber 102 andfinally pushes the diaphragm of the chamber 102, moving needle 105against vessel 104 to open it. Thereafter, when the screw 110 is rotatedin the opposite direction, the boss 109 is withdrawn to the end of thecase 107 and the bellows 108 is compressed, thereby returning to theinitial condition. In this case, the diaphragm and needle of the chamber102 are also restored to the initial condition and thus the contact withthe Cs vessel is broken. Thereby heat propagation to the Cs vessel fromthe outside is interrupted. Cesium escapes from the broken hole of theCs sealing vessel, and is vaporized by the Cs vapor generator 1. Thusthe chamber 102 is gradually filled with such vapor and then the Cs atombeam is expelled into the magnetic field A from a collimeter in the beamgenerator vessel 1 at a speed determined in accordance with thetemperature. The Cs atom beam generator 1 includes other parts notillustrated, such as a heater which maintains the cesium at the desiredtemperature and pressure, a temperature measuring thermistor whichmaintains and controls the temperature at a constant value, and getterwhich absorbs unwanted gas and maintains the vacuum condition. Anelectrical terminal 111 for these parts is provided at the spacer ring106 and extends to the outside.

The cylinder 204 of a magnetic field unit as shown in FIG. 5 isconnected to the detector vessel 701 which houses the detector 7, asshown in FIG. 3. The detector 702 is positioned within vessel 701 toreceive the atomic beam. Said detector 702 is assembled on a ceramicstem 703 sealing the end of vessel 701, which also houses parts notillustrated such as a hot wire ionizer, collector, and getter. Theterminal pins of these parts are extended through the stem 703.

For the purpose of exhausting the air from the atomic beam device toachieve a vacuum, the exhaust tube 704 is provided at the side ofdetector vessel 701 and it is chipped off after completion of airexhaustion. The location of the exhaust tube 704 is not restricted tothe detector, and it can be positioned elsewhere, for example, at themetal block of the high frequency transition part.

The above detailed explanation is related mainly to the structure formaintaining the vacuum condition of the atomic beam path, including theCs atom beam generator, the magnetic pole structure for the magneticfield A, the high frequency transition block, the magnetic polestructure for the magnetic field B, the detector, and the envelope ofthe beam path connecting said parts. Suitable material for the vacuumenvelope, that is materials which are stable at high vacuum andtemperature and which are highly reliable, can be easily selected. Otherparts or members, such as permanent magnets, coils, and the yoke formagnetic field C, which are influenced by a high temperature or whichmay release gas after air exhaustion, are not included in the vacuumenvelope. Therefore when exhausting the envelope to the vacuum conditionafter assembly, it can be maintained at the baking temperature for asufficient period of time. The soldering and welding sequence outlinedabove for connecting each part can, of course, be modified to facilitateassembly. It is recommended that the alignment of the beam path afterassembling the elements arranged to the right of the Cs vessel 104, bevisually established by firing the hot wire ionizer in detector 702.After alignment, the vessel 104 and the elements to the left of it areassembled as shown in FIG. 3. The method for connection is not limitedto soldering and adequate methods may be employed, for example, electronbeam welding and soldering in a hydrogen atmosphere.

The external accessories shown in FIG. 3 are added to the main body ofthe Cs atomic beam device produced as discussed above. At the sidesurfaces H--H of the block portions 301' of the high frequencytransition part 3, the yoke 401, which is made of the material havinghigh permeability, is mounted as shown in FIG. 7 corresponding to thecross section 7--7 of FIG. 3. At the lower area of said yoke 401, thecoil 402 is wound for the magnetic field C. At the circumference of theflanges 305 provided at the respective end surfaces I--I of the block301, the magnetic shields 1001 and 1002 for protecting the magneticfield C are concentrically mounted.

The magnetic shields 1001 and 1002 are spaced by the spacing elements1003 and then fixed to the flanges 305 by means of screws (notillustrated). Moreover, at the ends of said shield 1002, frames 1505 areheld via the flanges 305, and the casings 1502, 1503 and 1504 aremounted on frames 1505. Elements 1003 and frames 1505, which arecomposed of separate elements, are mounted in combination from theradial direction. For the magnetic poles 201 and 202 of the magneticunits for fields A and B, the permanent magnets 207A and 207B aremounted from the outside. Magnetic shields 208A and 208B supported onflanges 206A and 206B, respectively, are provided for magneticallyprotecting said magnetic fields A and B. The magnetic shields 208A and208B are also composed of different segments and are combined formounting from the radial direction. Since the cylinders 205A and 205Bwhich support the magnetic units for fields A and B must also supportthe weight of the Cs atom beam generator 1 and detector 7, the outsidesof the flanges 206A and 206B are reinforced in such a way that thecircumferences of said flanges are fixed to the sides of said frames1505 by using plural screws 209. Employment of such configuration isvery convenient because when said screws 209 are selectively adjustedaround the flanges 206 and the spaces between the flanges 206 and frames1505 are changed, the inclination of the Cs atom beam in the magneticfields can be adjusted. The atomic beam device is completed when thecasings 1502, 1503, and 1504 are mounted to the frames 1505. The casings1502 and 1504 are provided with the holes for the cables used forconnecting the atomic beam device.

As is explained above in detail, it is one of the characteristics ofatomic beam device of the present invention that the elements of thevacuum envelope are directly related to the path of the atom beam.Therefore, the vacuum envelope can be configured by using materials orparts which are very stable in heat and vacuum. In addition, since themagnets, the coil for magnetic field C, and wiring are not providedwithin the vacuum envelope, problems resulting from the fact that theyare exposed to a high temperature during machining and air exhaustion donot occur. For this reason, air exhaustion is fast, and little gas isreleased thereafter, which ensures a sustained vacuum for a long periodof time. Furthermore, complicated structure and treatment for preventingdemagnetization of the permanent magnets due to the baking temperatureture are not required. Since a sufficient vacuum is maintained for asufficient period of time for the ordinary operation of the atomic beamdevice, a vacuum ion pump is not required in practice. However, if morereliability is necessary, it is of course possible to add the ion pump.

It is another notable characteristic of the present invention that thehigh frequency transition part is a rigid unit made from the metal blockand this unit is used as the basic supporting body for the other parts.Thereby, the device has a coaxial structure with the atom beam pathconsidered as the axis, does not require the standard mounting, ensuresvery high rigidity, and receives little deformation due to thermaldistortion and external mechanical force. In addition, since the deviceis made with reference to the atom beam path, adjustments fordetermining the beam path after assembly can be drastically simplified.Particularly, when the atom beam generator 1 is mounted last, thearrangement of the beam path at each element can be adjusted easily andvisual from the side of magnetic field A, with the ionizer of thedetector being activated. Moreover, since the coil for magnetic field Ccan be mounted outside the vacuum, ordinary wires can be used and a coilhaving the desired performance can be designed easily. If desired, thecoil can be easily adjusted. Since the yoke for the magnetic field C ismounted and fixed to the side of said rigid metal block, there is nodistance variation between the poles. Therefore, a uniform magneticfield can be provided for the entire transition part.

The shield case and yoke have a structure that generate little stressdue to thermal deformation and even if a residual magnetization occursduring mounting, it can be erased by applying an AC field from theoutside. Since the permanent magnets for magnetic fields A and B areprovided outside the vacuum envelope, magnets small in size and light inweight and having excellent performance can be used. Adjustment of themagnetic fields can be performed easily by observing the operatingcondition of the atomic beam device and by measuring its performance,and thereby the device of the present invention shows excellentperformance when used as an atomic, molecular, or other particle beamdevice.

The above explanation is provided for easy understanding of anembodiment of the present invention, and this invention covers othervarious applications and modifications.

For example, the metal block forming the high frequency transition partcan be formed from two or more blocks. In addition, for example, whenthe high frequency sealing window position is not limited only to thearea before the branching point but is provided near to the transitionpart after the branching part, the vacuum part of the microwavewaveguide, and the block part thereof can be reduced. Additionally theshield case, etc., can be mounted effectively without division intoseveral portions. Moreover, other various kinds of modifications can beconsidered.

We claim:
 1. An atomic beam device of the type including a first stateselection magnet part; a high frequency transition part; and a secondstate selection magnet part sequentially arranged along an atomic beampath in a vacuum envelope extending from an atomic beam generator to adetector, wherein the improvement comprises: said high frequencytransition part comprises a rigid metal block forming a portion of thevacuum envelope, and said first and second state selection magnet partsare coaxially supported by said metal block at either end thereof.
 2. Anatomic beam device as in claim 1, wherein each of the first and secondstate selection magnet parts comprises a pole piece unit including apair of pole pieces hermetically assembled to form a portion of thevacuum envelope.
 3. An atomic beam device of the type including a firststate selection magnet part; a high frequency transition part; and asecond state selection magnet part sequentially arranged along an atomicbeam path in a vacuum envelope extending from an atomic beam generatorto a detector, wherein the improvement comprises: said high frequencytransition part comprises a rigid metal block forming a portion of thevacuum envelope, said first and second state selection magnet parts arecoaxially supported at either end of said metal block, each of the firstand second state selection magnet parts being comprised of a pole pieceunit including a pair of pole pieces hermetically assembled to form aportion of the vacuum envelope and being hermetically attached to saidmetal block through a metallic cylinder, and further comprisingmechanical adjusting means cooperating with each metallic cylinder foradjusting the alignment of the beam path.
 4. An atomic beam device ofthe type including a first state selection magnet part; a high frequencytransition part; and a second state selection magnet part sequentiallyarranged along an atomic beam path in a vacuum envelope extending froman atomic beam generator to a detector, wherein the improvementcomprises: said high frequency transition part comprises a rigid metalblock forming a portion of the vacuum envelope, said block comprising atleast two hermetically sealed metal portions configured for forming abeam path and a high frequency waveguide communicating with the beampath, and said first and second state selection magnet parts arecoaxially supported at either end of said metal block.
 5. An atomic beamdevice as in claim 4, wherein said portions comprise two symmetricalmetallic blocks joined along a plane parallel to the beam path.
 6. Anatomic beam device as in claim 3 or 5, further comprising yoke means forproducing a magnetic field within the block, said yoke means beingpositioned outside of the vacuum envelope adjacent the sides of theblock.